Decoupling excitons from high-frequency vibrations in organic molecules

• 成果类型: Article
• 署名作者: Ghosh, Pratyush; Alvertis, Antonios M.; Chowdhury, Rituparno; Murto, Petri; Gillett, Alexander J.; Dong, Shengzhi; Sneyd, Alexander J.; Cho, Hwan-Hee; Evans, Emrys W.; Monserrat, Bartomeu; Li, Feng; Schnedermann, Christoph; Bronstein, Hugo; Friend, Richard H.; Rao, Akshay
• 署名单位: University of Cambridge; National Aeronautics & Space Administration (NASA); NASA Ames Research Center; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of Cambridge; Jilin University; Swansea University; University of Cambridge
• 刊物名称: Nature
• ISSN/ISSBN: 0028-3722
• DOI: 10.1038/s41586-024-07246-x
• 发表日期: 2024-05-09
• 关键词: light-emitting-diodes; energy

摘要

The coupling of excitons in pi-conjugated molecules to high-frequency vibrational modes, particularly carbon-carbon stretch modes (1,000-1,600 cm-1) has been thought to be unavoidable 1,2 . These high-frequency modes accelerate non-radiative losses and limit the performance of light-emitting diodes, fluorescent biomarkers and photovoltaic devices. Here, by combining broadband impulsive vibrational spectroscopy, first-principles modelling and synthetic chemistry, we explore exciton-vibration coupling in a range of pi-conjugated molecules. We uncover two design rules that decouple excitons from high-frequency vibrations. First, when the exciton wavefunction has a substantial charge-transfer character with spatially disjoint electron and hole densities, we find that high-frequency modes can be localized to either the donor or acceptor moiety, so that they do not significantly perturb the exciton energy or its spatial distribution. Second, it is possible to select materials such that the participating molecular orbitals have a symmetry-imposed non-bonding character and are, thus, decoupled from the high-frequency vibrational modes that modulate the pi-bond order. We exemplify both these design rules by creating a series of spin radical systems that have very efficient near-infrared emission (680-800 nm) from charge-transfer excitons. We show that these systems have substantial coupling to vibrational modes only below 250 cm-1, frequencies that are too low to allow fast non-radiative decay. This enables non-radiative decay rates to be suppressed by nearly two orders of magnitude in comparison to pi-conjugated molecules with similar bandgaps. Our results show that losses due to coupling to high-frequency modes need not be a fundamental property of these systems. A molecular design strategy for reducing the vibration-induced non-radiative losses in emissive organic semiconductors is realized by decoupling excitons from high-frequency vibrations.